The present invention relates to a constant-voltage circuit.
In, for example, the push-pull output circuit of an acoustic power amplifier, in order to correct for distortion due to a curve in the V.sub.BE - I.sub.C characteristic of a transistor, the circuit is designed to cause an idling current to flow so that the output transistor may be operated in the linear region of the V.sub.BE - I.sub.C characteristic.
The base-emitter voltage V.sub.BE of the transistor, however, changes at the rate of approximately - 2.3 mV/.degree.C for temperature changes. For this reason, the idling current is also changed largely by temperature changes. When the idling current decreases, a crossover distortion arises. Conversely, when the idling current increases, the efficiency lowers and a thermal runaway takes place. Thereefore, a variety of constant-voltage (or constant-current) circuits have been developed which normally prevent the idling current from changing in response to voltage fluctuations. These circuits have, however, demerits along with their merits. For example, a known circuit of a certain type provides for a gradual increase in the idling current as the supply voltage arises. With another circuit, although the idling current is held constant for supply voltages in a specified range, the idling current decreases for supply voltages which are too high.